Carbon nanotube films and methods of forming films of carbon nanotubes by dispersing in a superacid

ABSTRACT

A novel method of forming thin films of carbon nanotubes (CNTs) is described. In this method, carbon nanotubes are dispersed in a superacid solution and laid down on a substrate to form a conductive and transparent CNT network film. The superacid, in its deprotonated state, is an anion that has a permanent dipole moment. The superacid solution may be a pure superacid or have additional solvent. Preferably, the superacid solution does not contain an oxidizing agent. Novel, highly conductive and transparent CNT network films are also described.

RELATED APPLICATIONS

This application is a national stage filing and claims the prioritybenefit of PCT/US08/81394 filed Oct. 27, 2008 and also claims thebenefit of priority from provisional U.S. patent application Ser. No.61/000,578 filed on Oct. 26, 2007.

INTRODUCTION

Carbon nanotube (CNT) networks have the potential to replace transparentconductive metal oxides, such as indium tin oxide, in applications wheresolution processing and flexibility are important. CNTs can beformulated into dispersions and then applied by solution processingmethods to a substrate, yielding flexible nanotube networks with goodadhesion to a variety of substrates. The nanotubes networks of thisinvention are primarily two-dimensional in nature, extending frommicrons to meters in two dimensions and a thickness that ranges from afew nanometers to a few hundred nanometers in size.

The Figure of Merit (FOM) for a thin transparent conductor isproportional to its bulk conductivity and inversely proportional to itsabsorption coefficient. CNT networks are composite structures comprisedof a mat of CNTs. In this case, the conductivity and absorptioncoefficient are influenced by the inherent properties of the nanotubes,the network morphology, and the resistance between the nanotubes. Forexample, the bulk conductivity depends not only on the conductivity ofthe individual nanotubes, but also on the contribution from theresistances at various types of junctions. The bulk absorptioncoefficient is determined by the absorption coefficients of the speciespresent (in a typical case, CNTs and air) and the volume fraction ofeach component. Both the conductivity and the FOM are enhanced byachieving CNT networks with debundled morphologies and long tubelengths.

For a thin film, sheet resistance is often the most relevant measure ofconductivity. The sheet resistance is a function of both bulkconductivity and film thickness. The sheet resistance typicallydecreases with increasing film thickness.

Debundling and dispersing CNTs without destroying the electricalproperties of the nanotubes remains a challenge. The dispersion ofcolloidal particles in general depends on the attractive van der Waalsinteractions, repulsive or attractive coulombic forces, and repulsiveinteractions arising from solvation or adsorbed layers. Due to theirlong length, high anisotropy, and poor compatibility with solvents, theattractive interparticle potential between CNTs is large. CNTs readilyform aggregates of aligned tubes with a CNT-CNT binding energy on theorder of 500 meV/nm. A number of methods have been developed to disperseCNTs. Typically, these methods have produced dispersed large nanotubesbundles, with the individual tubes in the bundle having significantlyaltered electronic and/or geometric structure.

Covalent modification of the CNT has been used to achieve dispersion ina range of solvents, depending on the functionality. Under appropriateconditions, high degrees of debundling have also been observed. However,covalent modification is destructive; it disrupts the π-binding andintroduces saturated sp3 carbon atoms that significantly decrease theconductivity of the CNT.

Dispersing agents such as surfactants, polymers, and non-covalentdispersing agents have been used to prepare debundled CNT dispersions.These agents must be removed after film formation in order to achievehigh conductivity. Rinzler et al. in U.S. Published Patent ApplicationNo. 2004/0197546 describe a method that removes the majority ofsurfactant during a two-step process to produce CNT networks. For someapplications, it is desirable to have a formulation that is suitable fora direct deposition process such as spin-coating, dip-coating,draw-down, screen-printing, etc.

Virtually all non-covalent dispersing methods require sonication toachieve dispersion. Sonication is known to cut the CNTs, shorteningtheir lengths. CNT films made with shorter tubes are predicted to havehigher resistance for an equivalent weight loading. Shorter nanotubeshave a lower aspect ratio, and therefore require a higher loading toachieve a percolating network. Other damage may be done duringsonication, as well. Studies suggest that prolonged sonication increasethe amount of amorphous carbon or the amount of defective CNTs present.In preferred methods of the present invention (described below),sonication is not used to disperse CNTs.

Many dispersing methods such as those employing aqueous surfactants andamines yield a distribution of dispersed bundles and single tubes.Centrifugation at high gravity is needed to separate the de-bundled CNTsfrom bundled nanotubes. Centrifugation requires expensive capitalequipment and results in low overall yields (<10%). The processgenerally produces dispersions with only very low concentrations ofnanotubes (<100 ppm), and therefore any process using these dispersionswould require use of large amounts of solvent. U.S. Published PatentApplication No. 2006/0099135 describes the production of highconcentration dispersions in aqueous surfactant. However, the sonicationprocess used in this procedure shortens the CNTs to lengths of about 200nm or less.

Debundled dispersions, such as from aqueous surfactant solution, havebeen deposited at a small scale by spin coating or solvent evaporationonto silicon wafers and other substrates for analysis. Often, areas ofindividual tubes can be found by analysis methods such as AFM or SEM.However, during large scale film formation of conductive, percolatingnetworks greater than 5 nm thick, it is observed that the CNT networksare composed of bundled CNTs, often greater than 20 nm thick. Thisoccurs due to re-bundling of the CNTs during film formation. There is aneed to a dispersion process and film formation method that maintainsthe debundled state in the network film.

Strong acids have been shown to readily debundle and disperse SWNTs.Most studies with CNTs and acid have conducted under conditions thatoxyfunctionalize, carboxylate, sulfonate, or nitrate the CNTs. Shibutain U.S. Pat. No. 5,853,877 describes a method of producing a debundledCNT network by the treatment of CNTs with strong acids that includesulfur, i.e., sulfuric acid and sulfur-containing superacids, inconjunction with an oxidizing agent, such as nitric acid, nitrate,permanganate, chromic acid, chromate, hydrogen peroxide and leaddioxide.

Smalley et al. have described neat superacid systems for producingmacroscopic fibers (U.S. Published Patent Application No. 2003/0170166)and alewive structures (U.S. Published Patent Application No.2003/0133865). Carbon alewives (named after a similar-looking fish) havea needle-like shape and are substantially free of tangles and longropes. In the latter publication, water was used to precipitateaggregates of CNT alewives. In US 20030170166, Smalley et. al. reportedthat the use of an oleum superacid, without using water to coagulate theCNTs, resulted in the formation of ropes of 200 to 400 nm in thickness.These thick ropes are undesirable for making transparent and highlyconductive films.

SUMMARY OF THE INVENTION

The present invention overcomes problems and disadvantages associatedwith current formulations and processes for producing thin conductiveand/or transparent CNT networks. The chemical and electronic structureof the CNTs is not disrupted by the chemical treatment or process. Themethod does not require sonication to achieve dispersion. Centrifugationis not necessary to achieve de-bundling, but may be used optionally topurify residual impurities in the sample. Formulations can be preparedat high concentration and in a 100% volatile system, thereby allowingapplication of CNT network films by standard processing methods such asspin-coating and screen printing. Highly debundled CNT networks can thenbe deposited on polymeric, glass, ceramic, silicon, composite, or othersubstrates.

Our invention relates to the use of reversible charge transfer reactionsthat allow debundling and dispersion in solution, maintenance ofdebundled state during CNT network formation, formation of networkstructures conducive to high film conductivity, and retention of theoriginal CNT structural and electrical properties after drying. In thismethod, a reversible charge transfer complex is formed that solubilizesthe CNTs in non-nucleophilic solvents.

It has been previously shown that oxidizing conditions (such as nitricacid) can covalently modify CNTs. We have surprisingly discovered thatexcellent dispersion for CNT network films can be obtained underconditions where side reactions leading to permanent covalentmodification of the nanotube are avoided.

In order to overcome the large van der Waals interactions thatdestabilize dispersions and cause re-bundling during film formation, arepulsive force is introduced by forming a complex between a charged CNTand non-covalently bonded charged species. This complex is electricallyneutral, and no net coulombic force exists between charged particlesseparated by large distances. At shorter distances, the diffuse portionsof the double layers interpenetrate, giving rise to a repulsiveinteraction. The distance over which this overlap occurs depends on thethickness of the double layer and the surface potential.

The CNT can be converted to a positively charged species though reactionwith a strong acid. Suitable acids include Bronsted superacids such aschlorosulfonic acid and trifluoromethanesulfonic acid, Lewis superacidssuch as antimony pentafluoride, conjugate Bronsted-Lewis superacids suchas tetra(hydrogen sulfato)boric acid sulfuric acid, and carborane acids.Strong oxidizing acid combinations such as nitric acid/sulfuric acidshould be avoided.

A key challenge to preparing thin CNT films from superacid dispersionsis controlling the demixing of CNTs from the solution during filmformation, which controls CNT-CNT aggregation and facilitates formationof debundled, yet strongly aggregated network. The demixing process isinitiated by the use of an organic solvent, which destabilizes theCNT-superacid charge transfer complex. The interaction parameter betweenthe solvent and CNT should be poor; otherwise, the solvent becomesincorporated into the CNT network phase, usually between junctions orwithin bundles, and lowers the quality and properties of the film.

It has been previously shown that washing a film cast from strong acidswith water results in an alewive structure. We show that washing withthe proper organic solvent results in a continuous film composed ofsmall bundles of CNTs and with strong interbundle interaction. Theinvention includes methods of preparing CNT films that includes anintermediate step of coagulating with a suitable organic solvent.

Considering the thermodynamics of a ternary CNT, superacid, and organicsolvent system, as well as the kinetics of its phase separation, a phasecan be created that contains primarily CNTs, but some residual superacidwithin bundles. Because the acid forms a strong interaction with the CNTvia a charge transfer complex, rapid coagulation with a poorlyinteracting organic solvent will remove excess acid, but will not removethose acid molecules directly associated with the nanotubes. Thisprocess results in a film where the nanotubes are effectively dopedduring the film formation step, eliminating the need for a separatedoping process to achieve high film conductivity, as is required forfilms formed by other methods.

In a first aspect, the invention provides a CNT network film,comprising: a tangled mass of CNT bundles in the form of a film disposedon a substrate. The CNT bundles have a mass average diameter of 20 nm orless, and have a mass average length of 500 nm or more, and have goodfiber-to-fiber connectivity such that the film exhibits a sheetresistance of 5000 ohms/square or less. Note that sheet resistance isthe best measure of fiber-to-fiber connectivity, and is necessary todefine “good” connectivity.

In another aspect, the invention provides a method of forming a CNTnetwork film, comprising: dispersing CNT bundles in a superacid to forma liquid composition of dispersed CNTs; depositing liquid composition ofthe dispersed CNTs onto a substrate; and removing the superacid. Thesuperacid either has a deprotonated anion, and the deprotonated anionhas a permanent dipole moment greater than zero; or if the superacid isa lewis acid that does not have a proton, then lewis superacid has apermanent dipole moment greater than zero. The superacid solutioncomprising the dispersed CNTs contains less than 0.001 weight %oxidizing agent, based on the weight of the CNTs in the dispersion. Thestatement that “the superacid has a deprotonated anion” does notnecessarily mean that the deprotonated anion is present in solution, butis used to define the symmetry of the superacid. Typically, however, insolution the deprotonated anion dominates over the protonated form, forexample in a pure Bronsted superacid or a conjugate superacid. In somepreferred embodiments, the deprotonated anion has a permanent dipolemoment of at least 1.0 debye.

An “oxidizing agent” degrades the structure of the CNTs and has acidityin sulfuric acid or a superacid containing sulfur. Examples of oxidizingagents include nitric acid, fuming nitric acid, nitrate (e.g. potassiumnitrate, sodium nitrate, etc.), ozone, permanganate (e.g. potassiumpermanganate, sodium permanganate etc.), chromic acid, chromate,hydrogen peroxide and lead dioxide.

The invention also includes CNT network films formed by any of themethods described herein.

One process for preparing films in this invention involves: selecting aCNT, forming a charge transfer complex with superacid, dispersing in adispersing medium, forming an initial CNT network film on a substrate(the substrate can be nonporous or could be a filter), and coagulatingthe debundled CNT structure using a nonaqueous non-solvent.Alternatively, the last two steps can be carried out simultaneously.

Any of the inventive aspects can be further described by any of thecharacteristics mentioned in any of the descriptions herein. Forexample, in preferred methods, the liquid composition does not containsulfuric acid. The methods can further include a step of transferringthe film to a second substrate. In some preferred embodiments, thesuperacid solution comprising the dispersed CNTs contains less than0.001 weight % or less than 0.0001 weight % oxidizing agent based on theweight of the CNTs in the dispersion. Preferably, the G/D ratio beforeand after treatment is less than 30%. The methods may further include astep of washing the CNTs with a nonsolvent such as diethyl ether ornitromethane.

Another preferred feature and a significant advantage of the methods ofthe present invention is that a highly conductive and transparent filmcan be obtained without the need for additional doping steps. After thefilm is formed, the CNTs do not need to be treated with a doping agent.This surprising result is shown in FIG. 10.

The network morphology produced by this method has a unique composition,wherein the network is composed of: (1) bundles with an average size of20 nm or less, preferably 10 nm or less, more preferably 5 nm or less,in some embodiments 3 nm or less, and in some embodiments in the rangeof 2 nm to 10 nm; (2) a high degree of isotropy in the plane of the film(that is, the CNT fibers generally lie in a plane); (3) these relativelythin bundles are primarily composed of CNTs longer than 500 nm,preferably at least 1000 nm in length; (4) strong interbundleinteractions as measured by low resistance; and, optionally, (5) chargetransfer doping within bundles. The result is network films withexceedingly high bulk conductivity, greater than 5,000 S/cm, preferablygreater than 10,000 S/cm, and more preferably greater than 13,000 S/cm.Films preferably have high transparency and low sheet resistance, forexample, 98% T at 550 nm and 1100 Ω/square. Throughout this description,“average” means mass average based on total mass of CNTs and excludesnon-CNT material such as the carbonaceous clumps that can be seen inFIG. 3.

In a further aspect, the invention provides a CNT network filmcomposition, comprising: a substrate, and a CNT network film on thesubstrate; wherein the CNT network film has a transparency of at least50%, a sheet resistance of 5000 ohms/square or less, and a G/D ratiobased on the average of Raman excitation bands at 532 and 633 wavenumbers of at least 15. The CNT network film has a bulk conductivityexceeding 11,000 S/cm. Preferably the CNT network has an area of atleast 1 cm×1 cm and the sheet resistance is measured on a square of 1 cmby 1 cm.

In another aspect, the invention provides a CNT network filmcomposition, comprising: a substrate, and a CNT network film and aliquid on the substrate; wherein the liquid comprises a superacid.Either the superacid has a deprotonated anion, and the deprotonatedanion has a permanent dipole moment greater than zero; or if thesuperacid is a lewis acid that does not have a proton, then lewissuperacid has a permanent dipole moment greater than zero. This aspectis similar to the corresponding method described above and can have anysimilar features. For example, in a preferred embodiment, the liquidcomprises a superacid and less than 0.001 weight % oxidizing agent,based on the weight of the CNTs in the CNT network film. The superacidshould be volatile. Note that this composition is an intermediate thatcould be subsequently applied to a substrate.

The CNT network films can be further characterized by various filmproperties. Preferably, the films have a transmittance of at least 80%,more preferably at least 90%. Note that transmittance (also termedtransparency) is measured at 550 nm unless specified differently. Sheetresistance is the best measure of fiber-to-fiber connectivity. Sheetresistance, if possible should be measured on a square having dimensionsof 1 cm×1 cm. In some embodiments, the film has an area of at least 1cm×1 cm, in some embodiments an area of at least 2 cm×4 cm, or at least8 cm×6 cm; and in some embodiments, the film has an area (where arearefers to geometric area, not surface area) of at least 1 cm², morepreferably at least 8 cm², and in some embodiments, in the range of 1cm² to 1000 cm². Preferably, the CNT bundles have good fiber-to-fiberconnectivity in the CNT network such that the film exhibits a sheetresistance of 1000 ohms/square or less, more preferably a sheetresistance of 500 ohms/square or less, and in some embodiments, a CNTfilm exhibits a sheet resistance in the range of 200 to 3000ohms/square. The films can be characterized by the data describedherein, and can also be described based on any combination of propertiesdescribed herein. For example, the films can also be described by thedata shown in FIG. 1; films can be described by the function % T=9×log(Resistivity)+66, where “log” is base 10, and this function can be usedto describe either an upper or lower bound of film properties, or anapproximate value. The films can also be described by their G/D ratiobased on the average of Raman excitation bands at 532 and 633 wavenumbers, preferably the G/D ratio is at least 15. The CNT network filmscan also be described in terms of thickness; preferably the films have athickness of 45 nm or less, more preferably 30 nm or less, and in someembodiments, 15 nm or less, and in some embodiments, the films are atleast 10 nm thick.

GLOSSARY

The term “carbon nanotubes” or “CNTs” includes single, double andmultiwall carbon nanotubes. The invention is not limited to specifictypes of CNTs. Suitable carbon nanotubes include single-wall carbonnanotubes prepared by HiPco, Arc Discharge, CVD, and laser ablationprocesses; double-wall carbon nanotubes (DWNTs), single double triplewall carbon nanotubes, and multi-wall carbon nanotubes, as well ascovalently modified versions of these materials. The CNTs can be anycombination of these materials, for example, a CNT composition mayinclude a mixture of single and multiwalled CNTs, or it may consistessentially of DWNT and/or MWNT, or it may consist essentially of SWNT,etc.

Debundled CNTs are preferably composed primarily (the majority ofbundles, based on total mass of CNT bundles, are of this size) ofbundles that are five nanotubes wide or narrower, more preferably fewerthan 3 nanotubes wide or narrower.

A CNT network film is a phase comprising CNTs (primarily randomlyoriented in two dimensions) and superacid. The superacid is preferablypresent at contents of less than 5% by weight (based on weight of CNTsplus superacid), and exists primarily at interbundle junctions andbetween bundles, within interstitial spaces. A CNT network film canexist by itself, can be disposed on a substrate; it can also beencapsulated in a material such as a polymer, or sandwiched betweenlayers of differing materials.

As is typical of patent terminology, “comprising” means including andpermits other components. In any of the descriptions, the inventionincludes articles and methods where “comprising” can be replaced by themore limiting terms “consisting essentially of” and “consisting of.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of transmittance versus sheet resistance for HiPco SWNTnetwork samples prepared according to this invention.

FIG. 2 is an SEM image of as received SWNT-HiPco-SP CNT fibers.

FIG. 3 is an SEM image of a CNT network film of SWNT-HiPco-SP CNT fibersprepared according to this invention.

FIG. 4 is a Raman spectrum of SWNT-HiPco-SP obtained at 785 nmexcitation.

FIG. 5 is a Raman spectrum of as received SWNT-HiPco at 532 nm.

FIG. 6 is a Raman spectrum of SWNT-HiPco at 532 nm after treatment withchlorosulfonic acid.

FIG. 7 is a Raman spectrum of as received SWNT-HiPco at 633 nm beforetreatment with chlorosulfonic acid

FIG. 8 is a Raman spectrum of SWNT-HiPco at 633 nm after treatment withchlorosulfonic acid.

FIG. 9 is a Raman spectrum of SWNT-HiPco at 532 nm after treatment byprior art dispersion method (Comparative Example 3)

FIG. 10 is a Raman spectrum of SWNT-HiPco at 633 nm after treatment byprior art dispersion method (Comparative Example 3)

FIG. 11 is a derivative of weight loss curve from TGA of SWNT-HiPco thatwas dispersed in chlorosulfonic acid.

FIG. 12 is a derivative of weight loss curve from TGA of SWNT-HiPcoControl that was sonicated in aqueous surfactant solution (ComparativeExample 3).

FIG. 13 is a Dark-field STEM image of CNT film prepared by prior artSDBS surfactant method.

FIG. 14 is a Dark-field STEM image of CNT film prepared by the methodsof this invention.

FIG. 15 is a Bright-field TEM image of CNT film prepared by prior artSDBS surfactant method.

FIG. 16 is a Bright-field TEM image of CNT film prepared by the methodsof this invention.

DETAILED DESCRIPTION OF THE INVENTION

The morphology and structure of a CNT network are described by severalparameters. CNTs can be described by length and diameter. Prior artdispersion methods that use sonication, oxidation, and/or covalentmodification tend to damage the CNT structure. The dispersion processemployed in this invention, superacids, does not change the CNTstructure. Analysis of a network film described by this invention willshow that the sidewall perfection and the length of the CNTs is high.The length of the individual CNTs within the network can be determinedby TEM analysis. The mass fractions of CNTs in a sample can be measuredby SEM, TEM, or other appropriate analytical technique.

CNTs can also be described by bundle size. Rebundling is prevented inthis invention due to the CNT-acid charge transfer complex, whichestablished a large diffuse electric double layer. As the liquid, suchas superacid or superacid/solvent mixture, is removed during the initialCNT network formation, the diffuse portions of the double layersinterpenetrate, giving rise to a repulsive interaction between CNTs. Thedistance over which this overlap occurs depends on the thickness of thedouble layer and the surface potential. A high dielectric constantmaintains the debundled network structure. Analysis by SEM, TEM, AFM, orRaman spectroscopy provides characterization of the bundle size.

A third level of structure is described by the bundle characteristics.In general, CNTs are composed of a mixture of semiconducting andmetallic tubes. Bundles will contain a mixture of both according to thestatistics of the system. It is possible to make bundles have a moremetallic-like character through these use of dopant; generally moremetallic-like behavior is desirable for electrical conductivity.Dopants, such as p-dopants, are often added in a second step. In thisinvention, the dopants are the acids and part of the dispersing process.Evidence of p-doping can be determined spectroscopically, for example,by examining the optical absorbance spectrum. Depletion of filled statesby an electron acceptor results in bleaching of the van Hovetransitions, and evidence of p-doping by the subject coating. Evidencefor p-doping is observed by measuring the spectrum, and then comparingthe spectrum after treatment with an n-dopant like hydrazine, which willshow evidence of any bleached transitions.

The final level of structure is described by the organization andinteractions between bundles. The interactions between bundles providethe cohesion within the film. By analogy with polymer films, theinterbundle interactions are the “crosslinks” between bundles. Preferredstructures for good film formation and high electrical conductivity havestrong interbundle interactions at the junctions between bundles. Inthis invention, the interbundle interactions can be enhanced by using anon-solvent to coagulate the structure. The interbundle interactions arecontrolled by controlling the thermodynamics and the kinetics of thephase separation process that occurs upon addition of a non-solvent tothe dispersion or to the initial CNT network. The non-solvent should bemiscible with the acid, but a non-solvent for the CNTs. The non-solventis chosen such that the interaction parameter between non-solvent/CNT ismuch weaker than between either CNT/acid or acid/non-solvent. Thispromotes rapid aggregation in the debundled network state.

In the prior art, CNT superacids dispersions have been used with wateror dilute sulfuric acid as coagulating agents. We found that the use ofwater as a non-solvent was ineffective in producing cohesive, debundledCNT films. This is due to the strong interaction between water and CNTs.Water and other molecules that strongly interact with CNTs, adsorb tothe surface of the CNTs and decrease the interaction strength betweenCNTs. To our surprise, the addition of poorly interacting solvents suchas diethyl ether to initial CNT networks yielded cohesive, debundled CNTfilms. The strong, interbundle interactions in the films produced inthis invention are exemplified by the shrinkage in the network structurethat occurs upon addition to the appropriate solvent. The shrinkageresults in higher density networks with outstanding electrical andmechanical properties, as described in the Examples.

The inventive methods preferably avoid the use of water and, preferably,use a nonsolvent to coagulate the CNTs (this step can occur prior to,simultaneously with, or after dispersing the CNTs on a substrate.Non-solvents which are not suited include non-solvents with highcapacity for hydrogen bonding, such as water and alcohols. These can bedetermined from the hydrogen-bond component of their Hansen solubilityparameter. Simple ethers (diethyl ether, dipropyl ether, dibutyl ether)can be suitable, but ethers with multiple oxygens such as diglyme havetoo much hydrogen bonding. One criterion for determining thesuitability, or lack thereof, for a potential non-solvent is to examineits value of the hydrogen-bonding component of the Hansen solubilityparameter. We have found that δ_(h) should have a value below 15(MPa)^(1/2) for the for the material to be a potential non-solvent forthis invention.

The reactivity of CNTs towards acids depends on the diameter of thenanotube. The purity and the initial morphology also play a role inreactivity. In its broader aspects, the invention applies generally toall CNTs. The CNTs that may be dispersed by this method includestraight, bent, or branched single wall carbon nanotubes (SWNTs),straight, bent, or branched multi wall carbon nanotubes (MWNTs),straight, branched, or bent few wall nanotubes (FWNTs), and theirmixtures. CNTs with average diameter from 0.5 nm to 60 nm may be used.Given the differences in reactivity as a function of sidewall curvature,selection of an appropriate acid depends on the CNT feedstock. CNTs mayalso include ultra-long CNTs (>1 cm) and ultra-pure CNTs. The class ofpeapods, substitutionally doped CNTs, and filled CNTs may be used.

A Bronsted superacid is defined as an acid stronger than 100% sulfuricacid. A Lewis superacid is defined as an acid stronger than anhydrousaluminum trichloride. Suitable acids include Bronsted superacids such ashalosulfuric acids (for example, chlorosulfonic acid and fluorsulfuricacid) and perfluoroalkanesulfonic acids (e.g., trifluoromethanesulfonicacid), Lewis superacids such as antimony pentafluoride, arsenicpentafluoride, tantalum pentafluoride, niobium pentafluoride, conjugateBronsted-Lewis superacids such as tetra(hydrogen sulfato)boric acidsulfuric acid, and carborane acids such as CHB₁₁R₅Cl₆, CH B₁₁R₅Br₆, andCH B₁₁R₅I₆, where R is H or CH3. Preferred acids include chlorosulfonicacid and trifluoromethanesulfonic acid. The invention also includesmixtures of any of these superacids in any permutation. Strong oxidizingacid combinations that degrade CNTs such as nitric acid/sulfuric acidshould be avoided. Oleum is also not a suitable acid combination forthis invention.

Without being held to a specific theory, we theorize that there are twoimportant factors that determine the suitability of an acid or acidcombination for this invention. One is that the acid must not stronglyoxidize the nanotubes, to prevent the introduction of defects, whichlowers the intrinsic conductivity of the individual nanotubes. Secondly,the acid should have a permanent dipole moment. For protic acids,determination of the dipole moment should be when in the deprotonatedstate. When the acid solvates the individual nanotubes, the permanentdipole moment indicates that the acid will have a preferred orientationrelative to the nanotube. In deprotonated chlorosulfonic acid, thedipole lies along the S—Cl bond. If the preferred orientation of CSA is,for example, with this dipole normal to the nanotubes surface, with theCl atom away from the nanotube, when two solvated nanotubes come closetogether, there will be a strong dipole repulsion helping keep thenanotubes separated. This interaction will be absent with an acid suchas sulfuric acid or in an acid mixture such as oleum.

Again, not being bound by any specific theory, we believe that thedesired exhibited by compounds suitable for the present invention can inpart be expressed in terms of relative dispersion, polar, and hydrogenbonding components of Hansen type solubility parameters, and relativeoctanol-water portion coefficients (Log P). For example, suitablecompounds such as chlorosulfonic acid and triflic acid have overallHansen solubility parameters of 30-40, with polar fractional componentsof 20-30, compared to unsuitable compounds such as sulfuric acid, whichhas a much higher solubility parameter ˜70, with a higher value of thepolar component: ˜60. These solubility parameter manifest themselves insuch properties as surface tension, where chlorosulfonic acid andtriflic acid may be in the range of ˜100-200 dyne-cm, while sulfuricacid is only ˜30 dyne-cm, and Log P where CSA and TFA have estimatedvalues in the range of positive (+) 0.1-0.6, more soluble in the organicphase, while sulfuric acid is ×10 more soluble in the aqueous phase LogP=negative (−1). In preferred embodiments of the present invention, thesuperacid preferably has a Hansen solubility parameter of 40 or less, insome embodiments in the range of 30-40.

The CNT charge transfer complex must be dissolved in a dispersionmedium, such as a solvent. Suitable solvents for the CNT/acid chargetransfer complex include the neat superacid itself or mixtures ofsuperacid. In some preferred embodiments, the superacid is chosen fromthe family of volatile superacids such as chlorosulfonic acidtrifluoromethanesulfonic acid, or antimony pentafluoride. Preferredvolatile superacids have a vapor pressure of at least 0.1 atm at 100°C., more preferably at least 0.5 atm at 80° C.

Alternatively the CNT/acid charge transfer complex may be dissolved insolvents including low-nucleophilicity solvents such as liquid SO₂,SO₂ClF, and SO₂F₂; typical Friedel-Crafts solvents such as diethylether, nitromethane, and tetrahydrofuran; or deactivated aromaticsolvents such as nitrobenzene. The superacid/solvent combination musthave sufficiently high dielectric constant to maintain the large diffuseelectric double layer. In preferred embodiments, the solvent has lownucleophilicity and high dielectric constant. Preferably, the dielectricconstant is greater than 35, more preferably greater than 60.

Dispersions are prepared by mixing the CNTs with a superacid, andoptionally diluting with solvent, under an inert environment atpreferably at temperatures ranging from −78° C. to 120° C., depending onthe reactivity of the CNT. At higher temperatures the screening lengthis expected to increase due to an increase in the ionic strength, whichwill decrease the effective distance of the repulsive interaction. Thetemperature stability of the superacid must also be considered. Thetemperature and time of the mixing process should be controlled toinsure that sidewall damage to the CNTs is not incurred. The extent ofsidewall damage can be monitored by precipitating the dispersion into anon-solvent and analyzing the CNTs by Raman spectroscopy, NIRspectroscopy, or TGA, as described in Measurement Issues in Single WallCarbon Nanotubes. NIST Special Publication 960-19.

Additives may be incorporated for improved gloss, durability, or otherphysical and mechanical properties. Preferably, these additives areavoided to maintain the highest conductivity.

Initial CNT networks can be formed from the dispersion using knownmethods to produce CNT networks on substrates. For example, thedispersion can be passed through a porous membrane filter to form a CNTmat. The membrane filter must be compatible with the solventcombination. Suitable membrane filters include those made of alumina,glass, PVDF, PTFE, PEEK, and polypropylene.

If volatile superacids are used, films can be formed by directdeposition methods such as spin coating, dip coating, and screenprinting. Substrates must be selected that are compatible with acid.Suitable substrates include glass, PTFE, PVDF, polypropylene, Kevlar,PEEK, ceramic, and others that would be recognized as being compatiblewith the acid.

The formation of cohesive CNT networks can be carried out by adding anon-solvent to the CNT film. Preferred non-solvents are miscible withthe superacid and have poor interaction with the CNT. Volatile solventswith boiling point less than 80° C. (at 1 atm) are preferred.Non-solvents can include ethers such as diethyl ether or THF,nitrogen-containing compounds such as nitromethane, or ketones such asacetone and methyl ethyl ketone. The non-solvents preferably do notcontain water (they are non-aqueous).

The cohesive, CNT network films, once formed, are strong and can betransferred in a second step, if desired to other substrates such asglass, polyester, silicone, polyethylene, etc using methods known in theart. Examples include, but are not limited to floating, direct pressuretransfer, and pad printing.

In some embodiments, the CNT network has good cohesion, average bundlesizes less than 10 nm, and is p-doped. In some preferred embodiments,the sample contains 0.01 wt % or less of S.

In preferred embodiments, water and other competing bases or reducingagents should be avoided during dispersion and film formation, in orderto prevent nanotube aggregation. Adventitious moisture in the air doesnot dramatically cause bundle formation on the time scale of the filmformation process, but film formation under nitrogen, argon, or otherinert gases is preferable. Decomposition of the CNT cation topermanently oxidized species can be avoided by maintaining temperaturesbelow 120° C. during dispersion of the CNTs and, also, preferably,during the film-forming process. Preferably, no sonication is used inthe inventive methods.

The following examples describe this invention in greater detail, anddemonstrate its advantages over the prior art. One important feature ofthe disclosed method is that the network morphology created is unique inthat it is composed of long CNTs (greater than 500 nm), organized intovery small bundles (preferably 20 nm or less, more preferably 10 nm orless, and still more preferably 5 nm or less), and with stronginterbundle cohesion. The inventive process promotes film formation ofCNT networks. This is demonstrated by comparing examples using differentnon-solvents. The unique morphology yields films with outstandingelectrical conductivity and low sheet resistance for films thinner than5 nm. This is demonstrated by measuring the sheet resistance and opticaltransmission of thin films deposited on polyester substrates. The use ofsuperacid disperses and debundles the CNTs without damaging them,cutting them, or fractionating them. This is demonstrated by comparingthe Raman spectra and thermal gravimetric analysis data of SWNTs beforetreatment and after treatment. To better differentiate the advantagesinherent in this invention, an example of a control sample prepared by astandard film formation method of sonication in aqueous surfactant isprovided. A second advantage of this invention is that superiorelectrical properties of the CNT film can be achieved at very lowloadings of CNTs.

EXAMPLES Sample Characterization

The sheet resistance of CNTs films was measured by the Jandel UniversalProbe and Corrware software. The Universal Probe consists of four 100-umtip radius pins, spaced 1 mm apart, aligned linearly. The films areplaced in contact with the pins under 25 g of spring pressure. Currentwas increased from −0.1 mA to 0.1 mA at a rate of 0.01 mA/sec. Allsamples exhibited ohmic response. The sheet resistance was determinedby: Rs=4.532×(V/I)×GFC, where V/I is the resistance, and the correctionfactor (GFC) is based upon film thickness vs. pin spacing. Given thethickness of these films, the correction factor is typically 0.99.Alternatively, the sheet resistance can be measured by painting twoparallel electrodes created with silver paint onto the CNT film alongopposite edges of 1″×1″ square. A digital multimeter can be used todetermine the sheet resistance.

Near infrared (NIR) and visible spectra of nanotube films were acquiredusing a Varian Cary 5000 UV/Vis/NIR spectrophotometer over the range of3000 nm (3333 cm⁻¹) to 400 nm. The spectra were acquired at a scan rateof 600 nm/minute with a spectral band width of 2 nm with an averagingtime of 0.1 seconds. In general, the samples were run against abackground of air. A blank substrate, not containing CNTs, as also runagainst air. The percent transmission was determined at 550 nm bycomparing the ratio of the transmission from the sample to the ratio ofthe transmission from the blank.

The Raman spectra of CNT films were acquired on a Horiba-JY “Aramis”Raman Confocal Microspectrometer operated in the backscattering mode at633 nm excitation. The sample was brought into optical focus with a 20×microscope objective. The Raman signal was optimized using the Real TimeDisplay feature with further adjustment of the Z-direction distance. Toinsure that no sample heating effects were observed, the laser power wasminimized to 5 mW at the sample. The stability of the position of theG-band can be used to analyze for heating effects. Data acquisitionemployed a 1-second acquisition time and 4 cm⁻¹ resolution; 16 spectrawere signal averaged. Spectra were acquired at several places on thesamples to obtain an overall average of the intensity of specific Ramanbands. The G/D ratio is determined by measuring the integrated intensityfor the D mode (1322 cm-1 at 532 nm excitation) and the G mode (1580cm-1 at 532 nm excitation). We also compared the ratio of the maximumintensity and found the result to be the same. A G/D ratio based on theaverage of Raman excitation bands at 532 and 633 wave numbers isdetermined by measuring the integrated intensity for the D mode and theG mode at both 532 nm and 633 nm and averaging these values.

Thermogravimetric analysis was carried out on a Perkin Elmer Pyris 1TGA. The temperature is calibrated using the Curie point transition ofPerkalloy and Alumel, the balance is calibrated using a 100 mg weightand the furnace is calibrated from 50-900° C. by the analysis software.Approximately 7 mg was loaded into Pt pan. The heating rate was 10° C.per min in air.

Film Formation

Example 1 Formation of Cohesive, Debundled CNT Films—Method A

Purified-grade SWNTs prepared by the HiPco process (SWNT-HiPco) wereobtained from Carbon Nanotechnologies, Inc. The SWNTs are reported tohave an ash content less than 15 wt %. A dispersion was prepared byadding 10 mg SWNT-HiPco to 50 mL of chlorosulfonic acid and stirring(using a bar magnet over a stir plate) at room temperature for 12 hours.A known volume of the dispersion was passed through a 0.2 μm Anodiscmembrane, with vacuum assistance, to create the initial CNT network ofdesired thickness. In some cases, the dispersion was diluted with moreacid to allow preparation of very thin films. Approximately, 50 mL ofdiethyl ether was added as a non-solvent, while maintaining vacuum. Thecolor of the SWNT network changed from yellow-brown to black uponwashing. The film may be observed to release at the interface betweenthe film and the Anodisc, which arises due to shrinkage caused from goodnon-solvent choice and the formation of cohesive interbundleinteractions. The SWNT network film was released from the membranefilter by dipping the coated membrane filter into a water bath. The SWNTnetwork film floated to the surface of the water and was picked up witha substrate such as polyester or glass. Visual inspection of a filmshows that the film is homogeneous.

The transmission of CNT coatings prepared by Example 1 and Method A weredetermined at 550 nm. The optical transmission is directly related tothe volume fraction of nanotubes in the coating and the thickness. Thetransmission versus sheet resistance of these samples is shown inFIG. 1. As expected based on a percolation model, the sheet resistancedecreases with increased transmission. Near the percolation threshold,it is expected that the sheet resistance will dramatically increase. Thevolume fraction at which percolation is achieved depends on the aspectratio of the nanotubes. Since the transmission versus sheet resistanceis fairly linear in the range tested, the loading is evidently wellabove the percolation threshold, consistent with the presence of longnanotubes that were not shortened by the dispersion process.

It should be noted that the results of further examples will be comparedto the data presented here. The results given in FIG. 1 can be roughlyrepresented as lying along a straight line. Films which have superiorperformance will lie above or to the left of this line, while films withpoorer performance will lie below or to the right of this line.

Example 2 Formation of Cohesive, Debundled CNT Films—Method B

A dispersion was prepared, as described in Example 1. A known volume ofthe dispersion was passed through a PVDF membrane filter, with vacuumassistance, and dried by passing a stream of dry air over the membraneto create the initial film. Diethyl ether was added as a non-solvent,while maintaining vacuum. The dried SWNT network film was placed incontact with a polymeric substrate such as Mylar and transferred to theMylar by applying pressure. Visual inspection of a film shows that thefilm is homogeneous.

Example 3 Formation of Cohesive, Debundled CNT Films—Method C

Direct deposition methods such as spin-coating and dip-coating may beused to prepare thin films on compatible substrates such as glass. As anexample, thin films were prepared by spin coating. Purified-grade SWNTsprepared by the HiPco process (SWNT-HiPco) were obtained from CarbonNanotechnologies, Inc. The SWNTs are reported to have an ash contentless than 15 wt %. A dispersion was prepared by adding 50 mg SWNT-HiPcoto 10 g of chlorosulfonic acid and stirring at room temperature for 120h. The resulting viscous dispersion was applied to a glass substrate byspin coating at speeds from 2,500 to 5000 rpm for 2 minutes under Aratmosphere. After drying, the coated glass slides were subsequentlywashed with diethyl ether. The sheet resistance of these samples was 176Ω/square and 110 Ω/square, respectively, and the transmission wasgreater than 60% at 550 nm. Spin-coating uses less solvent than otherprocessing methods and produces optical quality films with low RMSroughness. Visual inspection of a film shows that the film ishomogeneous.

Comparative Example 1 Use of Water as the Non-Solvent

Smalley et al. have described neat superacid systems for producingmacroscopic fibers (U.S. Published Patent Application No. 2003/0170166)and alewive structures (U.S. Published Patent Application No.2003/0133865). In the latter publication, water was used to precipitateaggregates of aligned SWNTs. A dispersion was prepared, as described inExample 1. A known volume of the dispersion was passed through a 0.2 μmAnodisc membrane to create the initial CNT network. 50 mL water wasadded as the non-solvent, while maintaining vacuum. Upon immersion intowater to float the sample, the films shredded and tore, rather thanreleasing, indicating poor interbundle cohesion and poor film formingproperties. Water cannot be used as a coagulant to prepare cohesive,debundled CNT films.

Comparative Example 2 Use of Oleum

A dispersion was prepared, as described in Example 1, exceptsubstituting oleum for chlorosulfonic acid. A known volume of thedispersion was passed through a 0.2 μm Anodisc membrane, with vacuumassistance, to create the initial CNT network of desired thickness.Approximately, 50 mL of diethyl ether was added as a non-solvent, whilemaintaining vacuum. The SWNT network film was released from the membranefilter by dipping the coated membrane filter into a water bath. The SWNTnetwork film floated to the surface of the water and was picked up witha substrate such as polyester or glass.

Visual inspection of a film shows that the film contains black specks.The black specks arise due to phase separation of the CNTs that occurredduring addition of the non-solvent. Cohesive, debundled networks filmscannot be prepared from oleum.

The sheet resistance of films prepared from oleum were higher then filmsprepared by Example 1. For example, a film prepared by the methodComparative Example 2 had a sheet resistance of 592 Ω/square and apercent transmission of 80.1%; as observed by comparison with FIG. 1,this value is to the right of the line, exhibiting poorer performance.

Example 3 Use of Different Non-Solvents

Films were prepared as in Example 1 with Method A, using differentnon-solvents, for diethyl ether

Solvent Result Ether Excellent film THF Good film Acetone Good filmMethanol Poor film Ethanol Poor film Isopropanol Poor film Diglyme Poorfilm Chloroform Poor film Nitromethane Excellent film Nitrobenzene PoorfilmThe suitability of a solvent for this invention is determined bymultiple factors, including among others its interaction with the CNT,interaction with acid, rate of evaporation, and hydrophobicity.Unique Morphology of Films of this Invention

Example 4 SEM and Raman of Super-Purified HiPco SWNT

The morphology of resulting CNT networks can be characterized by acombination of SEM and Raman spectroscopy. Super-purified gradenanotubes (SWNT-HiPco-SP) were chosen to demonstrate the high efficiencyof this method for producing debundled CNT networks, even from highpurity CNT sources. High purity nanotubes containing low levels ofresidual amorphous carbon often present the greatest challenge from astandpoint of debundling and dispersion.

Super-purified grade SWNTs prepared by the HiPco process (SWNT-HiPco-SP)were obtained from Carbon Nanotechnologies Inc. The SWNTs are reportedto have an ash content less than 5 wt %. A dispersion was prepared byadding 0.3 mg SWNT-HiPco-SP to 140 mL of chlorosulfonic acid andstirring for 3 h at room temperature and 48 h at 80° C. Coatings weredeposited on a polyester substrate using Method A of Example 1.

An SEM image of the SWNT-HiPco-SP before dispersion is shown in FIG. 2.An SEM image of a ˜100 nm thick CNT network prepared from chlorosulfonicacid dispersion is shown in FIG. 3. The powder is composed of denseaggregates composed of entangled super-ropes. After dispersion and filmformation, SEM shows that both the aggregate and super-rope structureshave been disrupted. The sample is composed of <10 nm bundles. Largequantities of spherical-like impurities are present in both samples.These can be removed by centrifugation of the dispersion prior to filmformation.

The radial breathing modes observed in the Raman spectrum are anotherindicator of the degree of debundling. The electronic dispersion of anindividual SWNT can be altered by aggregation. Development of electronicdispersion orthogonal to the usual 1D dispersion can cause bands to goout of resonance or go into resonance. For example, bands at 267 cm⁻¹and 204 cm⁻¹ arising from (10.2) and (13.2) type nanotubes come intoresonance at 785 nm when the nanotubes are bundled. The Raman spectrumof the coating prepared from highly purified HiPco SWNT is shown in FIG.4. Virtually no band is observed at 267 cm⁻¹, consistent with a highlydebundled SWNT sample.

Structure of CNTs in Films Produced by this Invention

Example 5 Treatment of SWNT-HiPco with Chlorosulfonic Acid

Purified-grade SWNTs prepared by the HiPco process (SWNT-HiPco) wereobtained from Carbon Nanotechnologies, Inc. The SWNTs are reported tohave an ash content less than 15 wt %. A dispersion was prepared byadding 10 mg SWNT-HiPco to 50 mL of chlorosulfonic acid and stirring atroom temperature for 12 hours. The treated SWNTs were collected bypassing the dispersion through a 0.2 μm Anodisc membrane and thenwashing two times with 150 mL diethyl ether. The powder was dried for 12hours at 110° C. and then for 24 hours in vacuum at 160° C. The samplewas subsequently characterized by Raman spectroscopy and thermalgravimetric analysis.

Comparative Example 3 Treatment of SWNT-HiPco by Sonication (Control)

Purified-grade SWNTs prepared by the HiPco process were obtained fromCarbon Nanotechnologies, Inc. The SWNTs are reported to have an ashcontent less than 15 wt %. A dispersion was prepared by adding 50 mgSWNT-HiPco to 50 mL of 0.62 wt % sodium dodecylbenzene sulfonate inwater and sonicating with a tiphorn (20 kHz and 225 Watts) for 30minutes. The SWNTs were collected by passing the dispersion through a0.2 μm Anodisc membrane and then washing with copious water until nosurfactant bubbles were observed. The powder was dried for 12 hours at110° C. and then for 24 hours in vacuum at 160° C. The sample wassubsequently characterized by Raman spectroscopy and thermal gravimetricanalysis.

The Raman spectra of SWNT-HiPco before and after film preparation asdescribed in Example 5 are shown in FIGS. 5-8. The Raman spectra ofSWNT-HiPco after film preparation as described in Comparative Example 3are shown in FIGS. 9 and 10. FIGS. 5, 6 and 9 are Raman spectra at 532nm excitation. FIGS. 7, 8 and 10 are Raman spectra at 633 nm excitation.Raman analysis is a useful indicator of the relative purity of thenanotubes. The relative ratio of the integrated intensity of the G modenear 1600 cm⁻¹ to the D mode near 1325 cm⁻¹ decreases with decreasingpurity. Damage to CNTs is accompanied by an increase in the intensity ofthe D mode. The integrated intensity of the D and G bands for thesamples at different excitation wavelength are shown in Table 1.

As shown in the Figures and the Table, the SWNT-HiPco before filmpreparation exhibit a relatively small D band associated with the purityof the as-received sample. The ratio of the G/D mode intensities isapproximately 24, depending on the laser excitation wavelength. Aftertreatment, the ratio of the G/D mode intensity is 23. More scatter isobserved as a function of laser excitation wavelength, but this iswithin experimental error of the method. The results suggest that thetreatment and process of this invention do not alter the chemicalstructure of the CNTs. The difference in the G/D ratio before and aftertreatment is much less than 30%.

The prior art dispersion treatment (e.g. Comparative Example 3) shows asignificant decrease in the G/D. The average is G/D is 20. Comparison ofthe spectra at 633 nm excitation shows evidence for the advantage of thesubject dispersion method over prior art methods. The contribution ofthe D band is much greater when sonication is used. The spectra at 532are more difficult to interpret due to the lineshapes caused by themetallic nanotubes.

TABLE 1 Results from Raman analysis of G and D modes of SWNT-HiPcobefore and after treatment. Excitation Intensity D Intensity G SampleWavelength Band Band Ratio G/D Before 532 42271 917987 21.7 Treatment633 26994 727593 26.9 After Treatment 532 27749 807388 29.1 by Example 5633 154716 2653570 17.1 After Treatment 532 75197 2083629 27.7 byComparative 633 148228 1733828 11.7 Example 3Thermogravimetric analysis is also an indicator of the level of damagethat is present in CNT samples. TGA (air at 10° C./min) of powdersbefore and after treatment both show an onset of decomposition near 420°C. The results suggest that little or nor damage was caused by thedispersion treatment. The first derivative of the weight loss curves forthe samples after treatment with chlorosulfonic acid is shown in FIG.11. For comparison purposes, the first derivative curve for a SWNT-HiPcosample prepared by prior art aqueous dispersion methods (e.g.Comparative Example 3) is shown in FIG. 12. This sample shows an onsetof decomposition of 358° C., suggesting that significant damage hasoccurred to the sample.

FIGS. 13 and 15 show STEM and TEM images for the CNT film prepared as inComparative Example 3. FIGS. 14 and 16 show STEM and TEM images forfilms prepared by the dispersion method of this invention. Comparison ofthese images reveals that the comparative treated film is much denserthan that treated with acid, that much thinner fibers/bundles areobserved in the films of this invention, and that in the films of thisinvention, the larger bundles are inter-connected by many thinner fibersor bundles.

Example 6 Effect of Centrifugation on CNT Film Properties

Although centrifugation is not required to obtain debundling, it may beoptionally used to improve the purity of CNT sources. Sphericalimpurities can be removed from rod-like impurities using mildcentrifugation. This is likely due to differences in the diffusioncoefficients of spheres versus rods.

Raw SWNTs prepared by Pulsed Laser Vaporization (SWNT-PLV) were obtainedfrom Oakridge National Laboratory. A dispersion was prepared by adding51.5 mg SWNT-PLV to 10 mL chlorosulfonic acid and stirred for 96 h. Theresulting viscous dispersion was centrifuged at 4000 rpm for 1 h. Thepellet and supernatant were individually collected, passed through a 0.2μm Anodisc membrane filter, washed with acetone, and the powders driedunder vacuum at 120° C. for 24 h. The powders were analyzed by Ramanspectroscopy and thermal gravimetric analysis. Alternatively, thesupernatant was diluted and used to prepare a film on Mylar using MethodA of Example 1.

Raman analysis of D and G modes of SWNT-PLV samples are shown in Table2. Analysis of the G-mode area to D-mode area indicates that thesupernatant is of higher purity than the initial sample and the pellet,suggesting that centrifugation is effective at removing carbonaceousimpurities from raw PLV tubes.

TABLE 2 Summary of Raman data to assess purity 532 G Mode D Mode G/DSample Area Area Ratio PLV-Raw 310107 22531 14 PLV-Supernatant 1921138748 22 PLV-Pellet 200664 36082 6The SWNT-PLV supernatant was re-dispersed, and transparent membraneswere prepared and transferred to polyester substrates. The sheetresistance was evaluated after drying in air for 48 hours, after dryingin vacuum oven for 24 h at 100° C., and again after exposing to air for48 hours. The transmittance was evaluated and compared to blanksubstrate. The results are shown in Table 3. The sheet resistance and %transmission of this example are similar to those of the resultspresented in Example 5.

TABLE 3 Results obtained for transparent conductive coatings preparedfrom raw SWNT-PLV supernatant. Sheet Resistance (Ω/□) After After After% T at 550 nm Sample Air Dry Vac Dry Air Expose (relative to PE)PLV-Supernatant 460 582 570 90.8

Comparative Example 4 Comparison to Prior Art Acid Dispersion

Mixtures of acid, including superacids, with an oxidizing agent such asnitric acid are often used for purification and light covalentmodification of nanotubes. Such dispersions have significantly lower FOMthan those of the subject invention. In this example, we prepared a CNTfilm on Anodisc using the method described in Example 1 with theexception that nitric acid was used in place of the chlorosulfonic acid.Raman spectroscopy showed that the G/D ratio of these films was lessthan 10, indicating that significant damage had occurred to the sample.

Preferred films of the present invention have a G/D ratio of 15 orgreater, preferably 25 or greater. In some embodiments, the inventivefilms have a G/D ratio similar to that shown in the Figures.

Properties with Different Types of CNTs

Example 8 CNT Films with Varied Types of Nanotubes

The invention is suitable for a wide range of nanotube types andpurities. Due to the differences in diameter, the conversion to CNTcations for different nanotube sources requires higher temperatures,longer dispersion times, or stronger acids. The conditions must beoptimized for each CNT source. Examples are shown below for severalsources; however the examples are not optimized cases, but shown simplyto demonstrate the applicability of the method to a wide range ofnanotube diameters and types.

SWNT-CVD

Purified, optical electronic grade SWNTs prepared by CVD were obtainedfrom Unidym. Films were prepared according to Example 1 and Method A.Coatings were deposited on PET.

SWNT-Arc:

Purified/Low-functionality grade SWNTs prepared by the Arc Dischargeprocess (SWNT-Arc) were obtained from Carbon Solutions. The SWNTs arereported to consist of 70-90% carbonaceous material and 7-10 wt % metalimpurities. A concentrate was prepared by adding 19.5 mg SWNT-Arc to 15mL of chlorosulfonic acid and stirring for 96 h at room temperature.Prior to use, the concentrate was passed through a plug of glass wooland diluted with chlorosulfonic acid by a factor of three.Alternatively, a coating was deposited on a polyester substrate usingMethod A of Example 1. A coating was deposited on a polyester substrateusing Method B of Example 2.

MWNT-CVD<10 nm:

MWNTs prepared by a CVD process (MWNT-CVD<10 nm) were obtained fromHelix Material Solutions. The MWNTs are reported to have an averagediameter less than 10 nm, a length from 0.5-40 μm, and a purity ofgreater than 95 wt %. A dispersion was prepared from ˜0.1 mg ofMWNT-CVD<10 nm and 10 mL of ClSO3H and stirred at room temperature for24 h. Coatings were deposited on a polyester substrate using Method A ofExample 1.

MWNT-CVD-35 nm:

MWNTs prepared by a CVD process (MWNT-CVD-35 nm) were obtained fromMaterials and Electrochemical Research Corporation. The MWNTs arereported to have an average diameter of 35 nm, 30 μm length, andpurity >90 wt % MWNT. A dispersion was prepared from ˜0.1 mg ofMWNT-CVD-35 nm and 10 mL of ClSO3H and stirred at room temperature for96 h. The dispersion was passed through a plug of glass wool prior touse. Coatings were deposited on a polyester substrate using Method A ofExample 1.

The sheet resistance and transmission for CNT films prepared fromdifferent sources is shown in Table 5.

TABLE 5 Results for different CNT sources Film Formation SheetResistance % T at 550 nm Source Method (Ω/□) (relative to glass)SWNT-CVD A 1243 98.4 SWNT-CVD A 60 90.9 SWNT-CVD A 20 71.6 SWNT-Arc A170 70.5 SWNT-Arc B 170 46.7 MWNT- A 1650 79.0 CVD < 10 nm MWNT-CVD-35nm A 3750 69.92The results in Table 5 indicate the outstanding utility of the method toproduce films. Films with extremely low sheet resistance can be preparedwith very high visible transmission. Correlation of the percenttransmission to film thickness indicates that SWNT with percenttransmission of 90% have film thickness of approximately 10 to 15 nm.The results indicate that the bulk conductivity of films produced bythis invention can exceed 11,000 S/cm and even 15,000 S/cm.Doped Structure

Example 9 Doping of Films Formed with Prior Art Dispersion Methods

It is well known that films formed by prior art dispersion methodsrequire a separate doping step, which serves to enhance the conductivityof the film. HiPco SWNT (CNI, Purified Grade) were dispersed in 0.62 wt% sodium dodecylbenzene sulfonate in D2O by tiphorn sonication. Theconcentration of nanotubes in the dispersion was 5.5 mg/L. Thedispersion (6 g) was passed through a 0.02 μm mixed cellulose esterfilter and washed with water. The resulting CNT mat was transferred toPET by: placing the CNT mat in contact with a sheet of PET, applyingheat (80° C.) and pressure (<10,000 psi) for 15 minutes, removing themixed cellulose ester from the PET by carefully peeling or swelling withacetone, and then washing the coated PET with acetone.

Samples were then immersed in a container of liquid thionyl chloride for2 minutes, maintained under moisture-free conditions. Samples wereremoved from the liquid, washed with methylene chloride, and then driedwith a stream of air. The film had 74% transmission at 550 nm. The sheetresistance before doping was measured to be 532Ω/□ and after was 133Ω/□.Comparison to the data of FIG. 1 shows that even when doped, the priorart films have a much lower FOM than films of this invention

Example 10 Doping Films of this Invention

In this example, two films were prepared as in Example 1, with theexception that after the wash step, one sample was immersed in acontainer of liquid thionyl chloride for 2 minutes, maintained undermoisture-free conditions. The sample was removed from the liquid, washedwith methylene chloride, and then dried with a stream of air. The sheetresistance of the two films was measured, and showed little differencebetween the two films. This supports the assumption that using the filmformation methods of this invention enhance the conductivity of thefilms via two mechanisms. One is the film formation methods of thisinvention create a CNT film that is more debundled and is has amorphology which promotes higher conductivity. The second is theat thefilm formation methods of this invention can lead to a film which isintrinsically doped during the film formation process, and requires nosecond doping step.

We claim:
 1. A carbon nanotube (CNT) network film, comprising: a tangledmass of CNT bundles in the form of a film disposed on a substrate;wherein the CNT bundles consist of CNT bundles having mass averagediameter of 20 nm or less and a mass average length of 500 nm or more,and wherein the CNT bundles have good fiber-to-fiber connectivity suchthat the film exhibits a sheet resistance of 5000 ohms/square or less.2. The CNT network film of claim 1 having a transmittance of at least80% measured at 550 nm.
 3. The CNT network of claim 1 wherein the CNTbundles have good fiber-to-fiber connectivity such that the filmexhibits a sheet resistance of 1000 ohms/square or less.
 4. The CNTnetwork film of claim 1 having a transmittance of at least 90%.
 5. TheCNT network of claim 4 wherein the CNT bundles have good fiber-to-fiberconnectivity such that the film exhibits a sheet resistance of 500ohms/square or less.
 6. The CNT network of claim 2 wherein the CNTbundles have good fiber-to-fiber connectivity such that the filmexhibits a sheet resistance in the range of 200 to 3000 ohms/square. 7.The CNT network film of claim 6 having a transmittance of at least 90%.8. The CNT network of claim 1 wherein the film has an area of at least 1cm×1 cm.
 9. The CNT network of claim 1 having a G/D ratio based on theaverage of Raman excitation bands at 532 and 633 wave numbers of atleast
 15. 10. The CNT network of claim 1 wherein the CNT bundles have amass average diameter in the range of 2 to 10 nm.
 11. The CNT networkfilm of claim 10 wherein the film has an area in the range of 1 cm² to1000 cm².
 12. The CNT network film of claim 10 wherein the CNT bundleshave good fiber-to-fiber connectivity such that the film exhibits asheet resistance of 200 to 3000 ohms/square or less.
 13. The CNT networkfilm of claim 1 wherein the CNT bundles have good fiber-to-fiberconnectivity such that the film exhibits a sheet resistance of 200 to3000 ohms/square or less; and wherein the CNT bundles have a massaverage length of 1000 nm or more.
 14. The CNT network film of claim 12having a thickness of 10 to 30 nm.
 15. The CNT network film of claim 1having a thickness of 10 to 45 nm.
 16. The CNT network of claim 1wherein the CNT bundles have a mass average diameter of 10 nm or less.17. The CNT network of claim 1 wherein the CNT bundles have a massaverage diameter of 5 nm or less.